In recent years, it has been proposed to install a radio communication module on the license plate of a vehicle. For example, electronic license plates are so designed as to be installed on a license plate and transmit vehicle information such as the information of the license plate by radio. (Refer to Patent Document 1, for example.) As a method for installing a radio communication module on a license plate, there are known methods of utilizing a bolt for installing a license plate on a vehicle body and fixing the module together with the license plate by tightening the bolt.
When a radio communication module is fixed by only such a bolt, it is preferable to separately provide a mechanism for suppressing the rotational displacement of the radio communication module relative to a license plate. One of methods for suppressing rotational displacement is such that a radio communication module is hooked on an end of a license plate and the module is thereby fixed. (Refer to Patent Document 2.)                Patent Document 1: JP-2000-289539 A        Patent Document 2: JP-2006-076345 A (US2006/0061456)        
However, these techniques involve a problem. It is difficult to install a module on a license plate with any other part present in proximity to an end of the plate or a largely deformed license plate. (Examples of the former license plate include license plates having a decorative frame and backlit license plates having a luminescent device at the back.)